Hornetsecurity experienced a minor incident on June 2, 2026 affecting Mail Traffic and Email Outbound and 1 more component, lasting 8h 18m. The incident has been resolved; the full update timeline is below.
Affected components
Update timeline
- investigating Jun 02, 2026, 02:48 PM UTC
We observe reports of emails being deferred when the receiving Host is a Microsoft Tenant. From our current Observations this seems to only affect German M365 Tenants. The behavior can be identified by following SMTP Code: 421 4.3.2 The maximum number of concurrent connections per resource forest has exceeded a limit, closing transmission channel Affected emails are in the deferred state. We are trying to deliver inbound deferred emails for up to 7 days and outbound emails we are trying to deliver up to 24 hours. Other services are not affected.
- monitoring Jun 02, 2026, 03:32 PM UTC
Microsoft is working to resolve the observed issue as soon as possible. More Details can be found here: https://admin.cloud.microsoft/#/servicehealth/:/alerts/EX1331830 We are monitoring the situation closely. Other services are not affected.